


Big Mistake

by Sepulchral_Sentience (orphan_account)



Series: Undertum But Not with Sans or Papyrus Because Others Deserve Love [1]
Category: Undertale (Video Game), undertum
Genre: Belly Kink, Experimentation, Gaster is Not a Skeleton, Gen, Instant Weight Gain, Male Weight Gain, Pre-Accident W. D. Gaster, Weight Gain Kink, male inflation
Language: English
Status: Completed
Published: 2021-01-07
Updated: 2021-01-07
Packaged: 2021-03-13 21:47:40
Rating: Teen And Up Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 1,771
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28535400
Author URL: https://archiveofourown.org/users/orphan_account/pseuds/Sepulchral_Sentience
Summary: By the power of science, Gaster explores the processing of monster magic- himself as the test subject. How wise.
Series: Undertum But Not with Sans or Papyrus Because Others Deserve Love [1]
Series URL: https://archiveofourown.org/series/2101365
Comments: 2
Kudos: 21





	Big Mistake

* * *

Sans was supposed to be the first pick for this sort of thing. Unfortunately, Gaster's loyal assistant was off for the day, coincidentally in the doctor's mind, since he'd merely _mentioned_ the need for a volunteer. He'd spooked him, Gaster realized with disdain, as he'd done before with several other 'breakthroughs' that usually winded up with Sans browsing for another, less-hazardous-to-his-health sort of job. The young monster had discussed going into the food business where nothing was poisoned with chemicals and he'd make faster bucks compared to the incorporeal paycheck of an assistant.

Gaster just thought he lacked a backbone- pun righteously intended. He never outright _harmed_ Sans, heavens no.. but he _did_ sometimes forget the skeleton had his own opinions about what went into his body and the side effects thereof. 

No matter. Today, on Gaster's own day off (as if a royal scientist ever had one of those) he'd spend it trying out his latest invention on the only person available. The doctor was quite stoic, already having performed questionable ventures on himself and others to the point some of it was admittedly kept in secrecy. Again, he never really _harmed_ anyone... 

And this certainly would _improve_ the livelihood of many monsters in the Underground. He was sure of it.

Lab coat swishing, the svelte doctor stepped through his grand laboratory and to a workspace. He tapped his holed hands on a computer keyboard. Somewhere on the other end of the room, a storage chamber was unlocked. Gaster ventured inside the walk-in fridge rolling with vapors, then returned with a vial. 

Fondly, he gazed over his glasses at the glowing blue bottle no bigger than his index finger, holding it up to the light.

Ah, his pride and joy.

This little wonder would ensure not only a full HP charge at the smallest dosage but an increase of magic produced as well. As known, monsters were composed of such essence, and recalling the War so long ago, to match a human in combat skill was impossible. At least, until now. Who knew, maybe this would be the one step closer to extracting DETERMINATION and using it for what his life long work was dedicated to...

Uncorking the vial, Gaster jotted down some notes on a clipboard. With a swift swig, the contents of the bottle disappeared down his throat. He licked his lips, then shuddered, his scarred face contorting amusingly. A note was jotted to make the formula appealing in taste. Sugary, he reckoned, to give that sort of brain boost some monsters like Sans were so addicted to.

 ***** **Gurgle...***

Surprised, the doctor raised his arms, smoothing a hand over his neat gray coat and sweater beneath. Hm, working already? With the help of a special scanner that resembled a coffee brewer, Gaster reviewed the analysis of his body- which was reacting perfectly to the formula. Already his limbs buzzed with energy, greater than normal. Also, he felt strangely, satisfyingly full, like he had finished eating a warm meal. Or one of the Queen's fresh pies, he thought with a chuckle.

His middle was some considerable degrees higher, a telltale sign of the magic being created and dispersed. According to his hypotheses and careful calculations, the formula would dissipate into his system and his HP would gain extra room, so to speak. In battle, he could last longer, move faster, think smarter-

The full, fuzzy feeling made Gaster set down his pen, hand caressing his forehead. New change, he noted. Starting to feel rather hot. When he scanned himself once more, his dark eyes narrowed, spotting a reading of an irregularity. Huh. Instead of dissipating, the magic seemed to be... replicating. How strange.

His tummy voiced itself unhappily. The gargling racket sent a wave of violet flush into the doctor's face, and he glanced around though no one else was occupying the lab. By Asgore's beard, that was louder than Sans before a lunch break. Rubbing his middle, he continued writing, scanning, logging the change that took a sudden twist he was _not_ expecting.

Gaster at first felt the fullness in his gut go past the adequate marker. Indeed he felt feverish, and that probably could be expected, what with the increased intake of magic he wasn't used to. But he suddenly felt _bloated,_ which didn't make much sense since the magic wouldn't necessarily _accumulate_ _-_

He winced at a jolt of passing pain emanating from his upset stomach. The royal scientist's thin digits held his concave side, which, to seemingly his imagination, ever so slightly curved outward. Gaster's soaring confidence in his project plummeted when he noticed his middle really _was_ pushing against his gray sweater. 

"What in the Underground-?" Typing quickly, scribbling furiously on his paper, Gaster reexamined his data. This was _not_ supposed to happen! Sure he'd expected this to be a work-in-progress, but this was hardly part of any malfunction he anticipated. As he searched for some sort of error in the planning, his belly proceeded to poke out, then round out, ironing his sweater snugly, then too much for comfort. Those noises rolling inside it sounded too much like a pipe churning with water, or maybe more accurately, a hose connected to a reservoir. 

The doctor bit his lip when a sharp coldness dug into his middle. Glancing down he saw a steadily swelling dome sitting upon his hips and waistband, kissing the table with vigor. It was dusted with glowing purple, magic clearly being born behind his navel. 

No, no, no, this was not good. Why was he getting bigger? Hadn't the small dosage of magic done its duty? Perhaps shortly, Gaster reasoned. But as his shirt rode up, exposing his snowy belly even further, doubts crept up with it. He should have had half a brain to make an antidote for emergencies like this. Well, what choice did he have now besides enduring what he’d subjected himself to?

Goodness, he was getting tight. His sweater was bunched around his magic-laden stomach, belt cinching unpleasantly. Gaster bit back a groan, fumbling around his expanding self to undo the constriction. No need; a second later the metal buckle almost punched another hole into his hand, zinging and pinging on the ground. His zipper followed suit. By now, it looked like he was gorging on basketfuls of the Queen's thank-you confections. Stumbling to the scanner, the doctor cursed at the reading of endless, self-replicating magic.

This didn't look like it would stop at all...

Panting, Gaster collapsed into the nearest seat, the cushiony comfort doing little to alleviate the building pressure he cradled in his lap. One glance and any monster would think he was pregnant with quadruplets. All he could think about was how Sans would have been in this unholy predicament- and how each unseen pump ballooning him up was a mixture of confusing, horrifying, yet... likable sensations. Heat traveled to and fro from his belly, and embarrassingly, lower. He'd done some brow-raising things, sure, but this was a first. Not being in control of his body, grasping his overburdened belly, and feeling it push his hands farther and farther apart...

Just when Gaster thought the burgeoning quantity of magic would be unceremoniously set free, the angry gurbling quieted. His hands felt the cramped surface of his skin shudder and cease activity.

Daring to open his eyes, the doctor slouched in his seat, breathing out in a rush. Well, that could have been disastrous. But what had caused this build-up? His estimations had been so clear. If he wasn't so overcome with shock, Gaster would have huffed in disappointment. Wiggling out of the chair, Gaster leaned over the table, writing down a brief note: Remember an antidote... and a will. He wasn't sure if the magic could even be used inside him; it felt trapped. Pent up. And in turn, he didn't feel very energized anymore. He was overwhelmed with lethargy as if he'd been filled with lead.

Very upset over its ordeal, his middle complained and ached with each movement. Gaster did his best to soothe the soreness, barely able to concentrate on reviewing the sequences on the monitor, trying to spot the source of this trouble. No good. There didn't seem to be any flaw besides... well, believing he could enhance a monster's system that fast. 

He'd learned his lesson; from now on anyone but himself would be used as experiment fodder.

Apparently, his belly didn't agree with the conclusion.

Beneath his palm, he began to blow up again. Gaster let out a shaky groan, not sure how much more of this- any of this -he could handle. This time, however, the quantities of his magic seemed to melt, thickening him, filling him out and turning into physical excess. 

"Fat," Gaster realized out loud, as the tensely-packed curve relaxed, plumping up into a pillowy mound. Of course. Increasing HP would just lead to having the magic be stored in places like the stomach. But how much had it increased? How much would need to be store-?

Seams crackled, and the doctor's hands flew to his backside in panic. Oh, Gods, no.

Along his pant legs came another splitting tear as his thighs went from slender stalks to tubby trunks, supporting the blooming shape of his doughy gut and love handles. His lab coat and sweater weren't faring well either, straining to contain a flabby set of cleavage. Every curve softened and swelled out further and further, wider and wider. The doctor's rear too had trouble staying modest and a distraught glance back showed two undignified halves of a full moon.

His fingers raked down the mass of paunch that engulfed space and enlarged heavily, fighting against the table's edge. Shivering, Gaster canted his plush hips upwards and panted shamelessly, feeling new parts squish, jiggle, and rub like he was one big hot marshmallow.

By the time his sagging breasts broke free of his sweater and his lab coat shredded, the magic was spent and stashed into layers of deliciously rotund lard.

A bright violet blush burned in Gaster's cheeks, which had also been infected by pudginess. Adjusting his askew glasses, he assessed as much of himself as he could. Scraps of clothing littered the ground, most of his uniform still keeping parts of him decent- if you could call clinging and showing off globes of purple-tinted flesh decent. _Gods,_ he was huge! How much magic had he _absorbed?_ There hadn't been that much in the vial to lead to this!

With a husky sigh, Gaster mumbled, "This was a mistake."

Oh well. Back to the drawing board... if he could reach it.


End file.
